Big variation between pumps too, 5psi on my joe blow is about 10 psi on my air tower.

If you want to just be good enough, then it's dead easy, fire in 35-40psi and get on with it. But if you want the best (for you) then there's no alternative but experimenting, which will mean some flats, but worth doing anyway.

I run mid-low 20s, because I can and I like it, and every so often I put a dent in a rim that looks like it's been hit with an axe Pros and cons eh.

I go by how far I can push the knobbles in to the tyre with my thumb.It's a measured amount of pressure I use...Of course as I've worked in hefty manual work for over 30 years my hand grip may well be a bit more than your average desk jockeys is.....

My track pump gives pressure readings that are way off but being a bit OCD a cheap pressure gauge from Halfords helps keep things consistent.

Same weight as you riding XC (i.e. riding anything and everywhere) and with tubes I would run 28psi front 32psi rear (as much as I could get away with without pinch flats) and drop that a bit more when running tubeless to around 26psi front 29psi rear. Probably could go lower tubeless but it starts to feel a bit "vague" on narrowish XC rims if I drop it any more.

Get the right tool to measure your tyre pressure repeatably then start experimenting

With tubeless, twice your weight in stone, minus one for the front, plus two for the back is a good starting point. Bigger tyres, wider rims and wet conditions (so less ultimate cornering force) allow you to go lower before you get annoying squirm or burping. If you're smooth and it's not too rocky this is a decent starting point for tubes too but you may pinch flat and have to go higher.

Er, doesn't this depend on the bike as well, ie rigid or suspension, wheel size and all that crap? If they squirm about, they are too flat, if you ping off every bump and rock and feel like you will die on every bend, well, they are too hard. If it feels good, go with it. As said already, gauges can be inaccurate. Use the same pump all the time if you can, so even if the psi gauge is wrong, at least you will be consistent!

It is helpful to know the rough pressure to avoid unnecessarily sliding all over the place or be constantly pinch flatting/ denting rims

Basic advice for the OP would be to experiment and make sure you are consistently measuring tyre pressure (i.e. consistent even if "wrong" readings) and experiment to find a middle ground that sits between the 2 conditions in the first paragraph

That's what Stan's recommend so it's not just me that finds it useful! But I do prefer wider rims and larger volume tyres - and all my pumps and gauges agree with each other. I can detect 10% variance easily - I don't know if that's a useful skill or just a hassle because I now check my pressures before most rides so I don't end up fiddling mid-ride...

The thumb test is useful too but it's deceptive as stiffer carcasses feel harder to the thumb than they do when riding - yes you can run lower pressures with tyres like that but not to the degree your thumb thinks!

Having just tweaked the pressures on a a Rubber Queen 2.2 UST and Butcher Control 2.3 this evening it was interesting to note how much firmer the 2.2 UST felt at equal pressure because the carcass is quite a bit bigger (despite the 2.2 vs 2.3 nominal widths) and the sidewalls stronger and thicker.